This invention relates to a grease composition for constant velocity joints used in motorcars, in particular for plunging type constant velocity joints and fixed type constant velocity joints, and more particularly, to a grease composition for constant velocity joints, which can effectively lubricate portions of such constant velocity joints which are liable to wear or cause abnormal vibration, to effectively reduce the friction of joints, to effectively reduce the occurrence of the vibration and to increase the life of the joints.
Recently, the number of front engine front wheel drive (FF-type) motorcars has rapidly increased for ensuring light weight and making interior space as large as possible. The number of functional four wheel drive motorcars (4WD) has also increased. Constant velocity joints (CVJ) suitable for the FF or 4WD motorcars have been used widely. Constant velocity joints are roughly classified into plunging type constant velocity joints and fixed type constant velocity joints. Typical plunging type constant velocity joints include double offset type constant velocity joints (DOJ), tripod type constant velocity joints (TJ) and cross groove type constant velocity joints (LJ). Typical fixed type constant velocity joints include Rzeppa type constant velocity joints (BJ) and undercut-free type constant velocity joints (UJ). Among the CVJs, an example of double offset type constant velocity joints (DOJ) used as plunging type constant velocity joints is illustrated in FIG. 1. In the DOJ, when it transmits torque at actuating angle, it causes complicated rolling and sliding motions in fitting between track groove 3 of outer ring 1, track groove 4 of inner ring 2 and ball 5 and accordingly, axial force is generated due to frictional resistance in plunging portions. The force is called induced thrust. Since the DOJ is provided with the track groove 3 in the inner surface of the outer ring 1 at 60 degree interval, six times of induced thrust are generated in every one revolution.
If the cycle of induced thrust generation agrees with the specific frequency of engine, body, suspension, and the like, sympathetic vibration is induced in the body, which gives the occupants uncomfortable feeling. Accordingly, it is desired to make the induced thrust as low as possible. Further, beating and/or confined noise are generated in a motorcar in high-speed driving. Moreover, as motorcars are made light weight and high power, the lubricating condition under which the DOJ is used gets much severer and therefore, it is necessary to increase the durability of the joints.
Conventional lithium soap thickened extreme pressure greases containing sulfur-phosphorus extreme pressure agents or lithium soap thickened extreme pressure greases containing molybdenum disulfide have disadvantages that they are low in vibration resistance and durability since they are liable to wear under high surface pressure. JP-A 62-207397 discloses an extreme pressure grease which comprises a sulfur-phosphorus extreme pressure agent which is a combination of molybdenum dialkyldithiocarbamate with at least one member selected from the group consisting of fats and oils sulfide, olefin sulfide, tricresylphosphate, trialkylthiophosphate and zinc dialkyldithiophosphate. However, the grease is not sufficient in giving quietness and durability.
Since the power transmission and steering are conducted through front wheels in FF and 4WD motorcars, constant velocity joints are necessary to ensure smooth power transmission even when a handle is fully steered. In general, fixed type constant velocity joints are used in the wheel side where lubricating condition is extremely severe due to high operating angle. Accordingly, the joints are liable to wear under high surface pressure and to thereby undergo damage in short period of time. In the past, lithium soap thickened extreme pressure greases containing sulfur-phosphorus extreme pressure agents or lithium soap thickened extreme pressure greases containing molybdenum disulfide have been used widely as a grease for constant velocity joints used in such high surface pressure conditions under which the joints are easily worn.